It has long been a desire and aim in the pacemaker art to provide a reliable single lead for use with a dual chamber pacing system. As is known, in the past a conventional dual chamber system typically has utilized two leads which interconnect the implanted pacemaker with the ventricle and atrium, respectively. Thus, in such a system a ventricular lead interconnects the pacemaker with the ventricle, delivering ventricular pace pulses to the ventricle and sensing ventricular conduction activity and returning such sensed signals to the pacemaker. A second, atrial lead is provided for performing the same functions with respect to the atrium. A long recognized disadvantage of this arrangement is the need to provide two leads, which adds to expense and increases reliability problems; and also takes significantly more physician time in placing the two leads at time of implantation.
The VDD or VDD(R) single pass lead provides a response to the two-lead problem, and has been in use for some time. In such a single lead construction, the distal tip has a fixation mechanism for fixing to the apex of the ventricle, while the lead is essentially "floating" or unattached in the atrium. The VDD lead is provided with one or two atrial electrodes, typically ring electrodes which are positioned in the atrium, for sensing P wave signals, thereby providing the ability to time out an AV delay and provide ventricular pace pulses which are synchronized to sensed atrial depolarizations. While some attempts have been made to pace from floating atrial electrodes, this has generally been ineffectual.
A further advance is what is known as the DDD lead, which has enhancements which aim to provide more stable contact with the atrial wall, so as to enable more reliable atrial pacing as well as reliable atrial sensing. The DDD lead typically includes features added to the lead portion to enable DDD operation, i.e., pacing and sensing in both chambers. The additional features are intended to maintain the atrial electrodes closer to the atrial wall when the distal tip is anchored to the ventricular apex. These features may include, for example, an atrial tine, or small extension from the lead body, which may be designed to provide better fixation against the atrial wall. The atrial tine may also be provided with a distal electrode, for making direct contact with the atrial wall. Other features which have been adapted to DDD-type leads include a variety of S-shaped leads and pre-shaped sections, for the purpose of providing more stable atrial positioning. See, for example, U.S. Pat. No. 5,628,778, Kruse; and U.S. Pat. No. 4,154,247, O'Neill; and "Towards Optimizing a Pre-Shaped Catheter and System Parameters to Achieve Single Lead DDD Pacing," PACE, Vol.20, May 1997, Part I.
There remains, however, a significant need to improve the ability of the DDD lead to accommodate different heart sizes, as well as heart movement due to ongoing contractions. Existing DDD leads are, to various degrees, not stable in the atrial area, do not adapt to heart size, and cannot move optimally with the heart during cardiac movement. There remains a significant need to adapt the DDD lead to heart movement and size, and to provide better anchoring with respect to the atrium.